It has long been known to manufacture and distribute pressure-sensitive adhesive stock for labels by providing a layer of face or facestock material for the label or sign backed by a layer of pressure-sensitive adhesive which in turn is covered by a release liner or carrier. The liner or carrier protects the adhesive during shipment and storage and allows for efficient handling of an array of individual labels after the labels are die-cut and matrix stripped from the layer of facestock material and up to the point where the individual labels are dispensed in sequence on a labeling line. During the time from die-cutting to dispensing, the liner or carrier remains uncut and may be rolled and unrolled for storage, transit and deployment of the array of individual labels carried thereon.
In many label applications, it is desirable that the facestock material be a film of polymeric material which can provide properties lacking in paper, such as durability, strength, water-resistance, abrasion-resistance, gloss and other properties. Generally, facestock material of thicknesses greater than about 3 mils have been used in order to assure dispensability in automatic labeling apparatuses. However, it is desirable to reduce the thickness or "down-gauge" the facestock material in order to attain savings in material costs. Such reduction in label thickness often has resulted in reduced stiffness and the inability to dispense the labels in a reliable commercially acceptable manner using automatic machinery.
Failure to reliably dispense is typically characterized by the label following the carrier around a peel plate without dispensing or "standing-off" from the carrier for application to the substrate. Such failure to dispense is believed to be associated with excessive release values, between the label facestock material and the liner. The release level also is dependent upon the stiffness of the facestock. Failure to dispense may also be characterized by the folding of the label due to lack of label stiffness at the dispensing speed as it is transferred from the carrier to the substrate. Another particular need in many labeling applications is the ability to apply polymeric-film labels at high line speeds, since an increase in line speed has obvious cost saving advantages.
Polymeric materials which have been suggested in the prior art as useful in preparing labels include polymeric biaxially-oriented polypropylene ("BOPP") which is relatively inexpensive and dispenses well. Such films tend to have sufficient stiffness for dispensing, but they also have relatively high tensile modulus values in both machine-direction (MD) and cross-direction (CD) which results in unacceptable conformability characteristics. Another useful material is unoriented polyethylene that is also relatively inexpensive and conformable. However, unoriented polyethylene generally is difficult to die-cut consistently and does not dispense very well.
In general, the prior art polymeric film labels have not been entirely satisfactory in die-cut label applications, particularly those involving polymeric-film materials less costly than "vinyl," i.e., polyvinylchloride (PVC). For example, down-gauging of polymeric-film labeling stock for improved economy has been inhibited by dispensing problems and the speed of label application lines has been limited when applying polymeric-film labeling stock less costly than PVC but still otherwise suitable for die-cut labels used on flexible or rigid substrates.
When the biaxially-oriented films are applied to rigid substrates such as glass bottles, the application is not completely successful due to the tendency of the relatively stiff label to bridge surface depressions and mold seams resulting from bottle-forming processes which results in an undesirable surface appearance simulating trapped air bubbles. This has impeded the use of pressure-sensitive adhesive labels to replace prior glass bottle labeling techniques such as ceramic ink directly bonded to the bottle surface during glass bottle manufacturing processes. Such ceramic ink techniques are environmentally undesirable due to objectionable ink components and the contamination by the ink of the crushed bottle glass in recycling processing.
The machine-direction-oriented labels of the present invention are to be contrasted with shrink-films consisting of stretched, unannealed films, sometimes used in sleeve-labeling applications wherein a sleeve or wrap of shrink film is placed around the circumference of a bottle or can or like container and heated to cause it to shrink into light, surrounding engagement with a container. Examples of such shrink film labels are found in U.S. Pat. Nos. 4,581,262 and 4,585,679. The tendency to shrink causes such film to tend to withdraw from any borders leaving exposed adhesive. The exposed adhesive presents a particular disadvantage in die-cut label applications since the exposed adhesive is unsightly and tends to catch dust.
Resinous film-forming materials which are blends of "soft" polar additives ("SPA") such as ethylene vinyl acetate copolymer (EVA) with low-cost olefinic base materials such as polypropylene, polyethylene, or combinations thereof including propylene-ethylene copolymers, blends of polyethylene and polypropylene with each other, or blends of either or both of them with polypropylene-polyethylene copolymer have been suggested as useful in preparing die-cut labels. In U.S. Pat. No. 5,186,782, extruded heat-set polymeric films are described which are die-cut into labels and applied to deformable or squeezable workpieces after being treated differently in their lengthwise and cross directions so as to have different stiffnesses in the respective directions. The polymeric films described in the '782 patent include heat-set unilayer films, and specifically films of polyethylene, as well as multilayer films which may comprise a coextrudate including an inner layer, a skin layer on the face side of the coextrudate, and optionally a skin layer on the inside of the coextrudate opposite the face side. A pressure-sensitive adhesive layer is generally applied to the inner side of the coextrudate. Preferred materials disclosed for use in the skin and inner layers comprise physical blends of (1) polypropylene or copolymers of propylene and ethylene and (2) ethylene vinyl acetate (EVA) in weight ratios ranging from 50/50 to 60/40. The core material also may be polyethylene of low, medium or high density between about 0.915 and 0.965 specific gravity.